BISMARCK, N.D. – North Dakota officials are trying to determine if Tesoro Corp. knew about potential problems – including one deemed “serious” in documents obtained by the Associated Press – with a pipeline that leaked more than 20,000 barrels of crude oil in a wheat field in the northwestern part of the state.

Dave Glatt, chief of the state Department of Health’s environmental health section, said Wednesday that regulators want to know more about inspections conducted before the spill reported by a farmer harvesting wheat on his farm near Tioga, N.D., on Sept. 29.

“We have heard they may have found some anomalies in the metal but not necessarily holes,” Glatt said. “We have heard there were some potential problems identified.”

Tesoro had not shared results of recent pipeline tests with health officials, Glatt said. Cleanup of the 7.3-acre spill area is priority with the state at present, he said.

“We want to know the integrity of the pipeline, what kind of monitoring was in place – and we will send a formalized letter asking that when we get a breather here,” Glatt said.

Farmer Steve Jensen discovered oil spill the size of seven football fields while harvesting wheat Sept. 29. Tesoro Corp. first estimated the spill at its underground pipeline near Tioga at 750 barrels. About a week later, the San Antonio-based company increased the estimate to 20,600 barrels, making it one of the largest spills in North Dakota history.

In a statement to the Associated Press, Tesoro said it inspected the pipeline about two weeks before the spill was reported, using a robotic device called a “smart pig” that travels through a pipeline to search for corrosion and other problems.

“We were awaiting results of the analysis of that inspection when the leak was reported,” Tesoro said.

Tesoro has said that the hole in the 20-year-old pipeline was a quarter-inch in diameter. Tesoro officials have not speculated on what caused the hole in the 6-inch-diameter steel pipeline that runs underground about 35 miles from Tioga to a rail facility outside of Columbus, near the Canadian border. State officials have said it may have been caused by corrosion.

Tesoro said it acquired the pipeline when it bought an oil refinery in Mandan in 2001. The company said it inspected the pipeline in 2005 and on Sept. 10-11 of this year. The company said the federal Pipeline and Hazardous Material Safety Administration inspected the pipeline in late 2010.

The agency remains closed because of the ongoing federal government shutdown.

The company on Wednesday did not immediately answer questions about its inspection results and why it sent crude through the pipeline without knowing them.

According to documents obtained by the AP, the state’s top oil regulator said in an email that Tesoro’s “results show a serious problem where the leak occurred.”

Department of Mineral Resources Director Lynn Helms has previously refused to comment on the spill, saying it wasn’t part of his agency’s jurisdiction. In an email to his daughter, a Penn State University student, Helms said the pipeline was not required to have “real time pressure monitoring and shut-down devices.” He said the company was in the process of “installing them anyway but started the pipeline without them because they weren’t required.”

Helms was out of the office and could not be reached on Wednesday, his staff said.

Other documents obtained by the AP say one state inspector, in an email to top health officials, said that one Tesoro official had told him the company “had the results of the pipeline inspection for several months.”

Scott Stockdill told his bosses in an email that his questions are “something that should be followed up on.”

Scientists who helped calculate oil spilled from a broken BP well into the Gulf of Mexico are questioning the methodology used by Tesoro to estimate the amount of crude that spilled in North Dakota.

Tesoro said it came up with its spill estimate using ground analysis. But oil spill experts say a more accurate assessment likely would come from calculating how much crude went into the pipeline versus what was supposed to come out at its terminus.

Tesoro said its “investigation included a thorough examination of the site spill characteristics including factors such as surface area and depth of soil impacted, and soil porosity.”

The company would not elaborate.

Purdue University engineering professor Steve Wereley said Tesoro’s calculation of how much oil it released likely is “at best, a guess.”

Wereley, who along with other scientists helped estimate the amount of oil spilling into the Gulf in 2010, said he was unaware of any scientific studies that could back Tesoro’s estimates.